Structure of a Gold(III) Hydroxide and Determination of Its Solubility

Abstract

A representative gold(III) complex ion ([AuCl4]−), successively hydrolyzes to form [AuCl4−n(OH)n]− (n = 0–4) in aqueous solution as the pH increases and, finally, precipitates. It has been referred to as “gold(III) hydroxide” for the past fifty years. However, whether the precipitate is gold(III) hydroxide (Au(OH)3) or hydrous gold(III) oxide (Au2O3·nH2O) has remained uncertain. Few studies have been conducted to determine its chemical and physical properties. The aim of this investigation is to identify the precipitate and to determine its solubility. The precipitate was X-ray amorphous. Based on the transmission electron microscopy (TEM) observation, the shape was spherical and the particle diameter was approximately 9 nm. From the results of 197Au Mössbauer spectroscopy, Au L3-edge X-ray absorption (XA) spectroscopy and thermogravimetry/differential thermal analysis (TG/DTA) measurements, the precipitate composition was estimated to be Au(OH)3. Based on the composition, its structure was proposed to be a linear polymer. Moreover, its solubility at 298 K was determined to be 0.00120 g/100 g of H2O. This chemical and physical properties information of Au(OH)3 is essential for gold chemistry, for example, for preparing supported gold catalyst.

Redefinition of the Active Species and the Mechanism of the Oxygen Evolution Reaction on Gold Oxide

“…Actually, gold(III) precipitations in alkaline media have been shown to correspond to Au(OH) 3 but not Au 2 O 3 •nH 2 O according to X-ray diffraction, transmission electron microscopy, Au Mossbauer spectroscopy, X-ray absorption spectroscopy, and thermogravimetry/differential thermal analysis studies. 39 It is interesting to note that the reduction potential difference of αand β-oxide is roughly 0.13 ± 0.01 V in both acid and alkaline solutions but becomes more than 0.4 V under neutral conditions (0.42 V at pH 6.8). Therefore, RDE experiments were conducted with varying rotation rates (Figure 3b).…”

“…From a comparison of cyclic voltammograms at different pH (Figure a), the β-peak therefore most likely corresponds to the reduction of Au­(OH) 3 . Actually, gold­(III) precipitations in alkaline media have been shown to correspond to Au­(OH) 3 but not Au 2 O 3 • n H 2 O according to X-ray diffraction, transmission electron microscopy, Au Mössbauer spectroscopy, X-ray absorption spectroscopy, and thermogravimetry/differential thermal analysis studies …”

Investigations on Adsorption of Inorganic Ions in Aqueous Solution to Some Metal Oxides, Hydroxides and a Carbonate by the X-Ray Spectroscopic Method

“…Thus, Kawamoto et al prepared Au(OH)3 and examined its chemical properties. Consequently, Au(OH)3 was found to be a suitable standard material for Au(III) in the measurement of Au L3-edge XAS data, 39 and Au(OH)3 is now used as a reference standard for Au(III). 36 4•5 What is the driving force for the adsorption of Au(III) complex ions on metal oxides and hydroxides?…”

Reaction Monitoring of Gold Oxides Prepared by an Oxygen-dc Glow Discharge from Gold Films in Various Aqueous Solutions by a Surface Plasmon Resonance-based Optical Waveguide Sensing System and X-ray Photoelectron Spectroscopy

“…It was found that the gold oxides do not decompose in these acid solutions at the same concentration as that of the hydrochloric acid solution. Therefore, it is believed that a water-soluble tetrachloroauric acid (H[AuCl4]) [34][35][36] is produced by the reaction of the gold oxide and the hydrochloric acid in the solution. Decomposition of gold oxides was also observed in 1.1×10 −3 and 1.1×10 −2 M hydrobromic acid solutions using XPS.…”

“…The gold oxide layer (0.65 nm thick) was found to decompose in a 1.0×10 −2 M aqueous sodium hydroxide solution and it decomposed completely after 22 min at room temperature.Decomposition of gold oxides was also observed in 1.0×10 −3 and 1.0×10 −2 M potassium hydroxide solutions using XPS. Gold hydroxide (Au(OH)3) nanoparticles were prepared from an aqueous solution of tetrachloroauric acid (H[AuCl4]) by raising the solution pH and the precipitate structure was characterized by electron microscopy, Mössbauer spectroscopy, X-ray analysis, and thermal analysis 36. The water solubility of Au(OH)3 at room temperature was reported to be 1.20 mg/100 g of H2O.…”